Nanostructures have recently been utilized in a variety of bio-sensing applications due to their enhanced surface area, precise biomolecule-electrode connections, and enhanced delivery of application agents. In the realm of electrochemical sensing, conductive nanostructures immobilized on electrodes enhance electrocatalytic behavior due to quantum confinement and may exhibit properties including more favorable Faradic-to-capacitive current ratios, higher current densities, and faster mass transport by convergent diffusion than their larger micro/macro electrode counterparts. In order to increase biosensor current output to measurable levels, large arrays of nanostructures (i.e., nanoelectrode arrays [NEAs]), have been immobilized on electrode surfaces. These NEA biosensors, fabricated with various nanostructures (e.g., nanowires, nanotubes, and nanocrystals) have shown promising results, displaying high sensitivities and fast response times.
Recently developed graphene petal nanosheets, with reactive edge planes similar to oriented pyrolytic graphite (HOPG) or vertically oriented CNTs, can be grown directly on a variety of surfaces without the need for metal catalysts—creating a nanostructured surface well suited for integration into numerous electrochemical sensing applications.
Various biofunctionalization techniques have been developed to immobilize biorecognition agents onto electrode surfaces including covalent binding through self-assembled monolayers (SAMs), non-covalent membranes, and electrodeposition with conductive polymers. Each biofunctionalization technique has advantages. Self-assembled monolayers provide a covalent link to the biorecognition agent and electrode surface. Non-covalent membranes can be rapidly assembled on electrode surfaces. Poly(3,4-ethylenedioxythiophene) (PEDOT and sometimes referred to as PEDT) is an electrically conductive polymeric material that can be utilized in biosensor interfaces due to its biocompatibility, stability, and high conductivity. Mixtures of the monomer 3,4-ethylenedioxythiophene (EDOT) and Poly(styrene-sulfonate) (PSS) are soluble in aqueous environments and can be controllably electrodeposited onto conductive surfaces. Furthermore PEDOT displays high stability with aqueous electrolytes. This high electrochemical stability, owing to inherent dioxyethylene bridging groups, makes PEDOT well suited for enzyme immobilization.
Water soluble molecules can also be incorporated into the PEDOT matrix during electropolymerization. PEDOT has been used as an enzyme immobilization matrix for use in glucose and cholesterol amperometric biosensing applications.
Carbon nanomaterials (e.g., carbon nanotubes, nanospheres, nanohorns, nanoplates, nanoparticles) have attracted considerable research attention due to their unique properties and potential applications. Transition metals such as Fe and Ni have been traditionally viewed as important catalysts for sp2 carbon growth since they enable rapid dissociation of carbon-rich molecules to form metal-carbon alloys that precipitate carbon through a vapor-liquid-solid mechanism. Two dimensional graphene in the form of single-layer graphene (SLG) or few layer graphene (FLG) has been the particular focus of much recent research because of its unique electronic properties.
In contrast to the production of conformal sheets of SLG or FLG, small crystalline graphitic petals (GPs), or carbon nanowalls (or nanosheets) containing a few layers of grapheme have interesting industrial applications because they grow roughly perpendicular to a substrate and dramatically increase the surface area from which they grow. The GPs are thin, containing only a few graphitic layers, and can be catalyst free, suggesting they might be a source of free-standing graphitic material. Various methods have been reported to grow GPs in the past decade, among which microwave plasma-enhanced chemical vapor deposition (MPCVD) is particularly common. GPs can be used for field emission enhancement, hydrogen storage, sensors, nano-composites and as a growth template for nanostructures of different materials.
In order to satisfy the requirements of today's increasingly multifunctional portable electronic devices, sustainable and renewable power sources, such as supercapacitors and batteries, are designed and fabricated in the trend of being small, thin, lightweight, environmentally friendly and even flexible. Electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, with the merits of high power density, fast power delivery or uptake and excellent cycle stability, have become some of the most promising candidates for next-generation high-performance power devices.
Due to high theoretical capacities, electrically conducting polymers (ECPs), such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTP), are commonly used as pseudocapacitive materials to further increase the energy and power density. Among them, PANI gains particular interests in the past 30 years because of its high theoretical specific capacitance (2000 F/g), high degree of processability and chemical stability in air, as well as its fairly high conductivity and favorable electrochemical cycling characteristics. In addition, PANI can also be synthesized in different morphologies (e.g., films, nanofibers, arrays) on different substrates. Despite of the high theoretical specific capacitance, ref. indicates that the current experimental value is far less than the theoretical one, because of the limited mass transport rates of anions and relatively low PANI conductivities. Therefore, it is essential to coat PANI on templates with a high specific surface area to fully exploit its electrochemical capacitive properties. Various porous carbon materials (e.g., carbon cloth, activated carbon, mesoporous carbon, and carbon nanotubes) were used as conductive templates.
Graphene, a new member of carbon nanomaterials with unique properties, was also combined with PANI to fabricate composites by in situ chemical or electrochemical polymerization, and self-assembly. In the most of the previous work, reduced graphene oxide was used as templates or supports for PANI nanostructures. Free-standing chemically converted graphene/PANI nanofiber paper-like composite was synthesized through vacuum filtration of suspensions of the two components. The composite shows a specific capacitance of 210 F/g and 160 F/cm3 but with a poor cycling life (21% loss at 3 A/g after 800 cycles). Reduced graphene nanosheets/PANI composite was synthesized using in situ polymerization in the graphene nanosheet suspension and a specific capacitance of 1046 F/g (based on GNS/PANI composite) was obtained at a scan rate of 1 mV/s. However, the specific capacitance shows a significant loss at 100 mV/s (˜50%) compared with that at 1 mV/s in the presence of conducting agent and binding materials.
Graphene nanosheets (nanowalls), or graphitic petals (GPs), containing a few layers of graphene and growing roughly perpendicularly to a substrate over a large surface area, are the ideal candidates for electrochemical energy storage applications, due to high specific area and high electrical conductivity. They were previously synthesized on different substrates, such as Ni foil and carbon cloth, for electrochemical energy storage application. The unique sharp edges of GPs greatly increase charge storage as compared with that of designs that rely on basal plane surfaces. Density functional theory analysis shows the presence of these edges affects not only the reactivity of the carbon material toward the adsorption of Li atoms but also their diffusion properties. Up to date, utilization of this highly conductive and unique GP structure as a nanotemplate to further exploit the electrochemical properties of the pseudocapacitive materials (e.g., conducting polymer) has rarely been reported, not to mention the applications of these composite electrodes in flexible two-terminal devices.
While in the application level of supercapacitors, all-solid-state and flexible supercapacitor devices, based on polymer gel electrolyte, have recently aroused particular interests in this research field because of their obvious advantages in environmental friendliness, flexibility, cost and versatility in comparison with many currently employed counterparts. The advantages of paper-like supercapacitors in structure design over conventional supercapacitor device configuration (a separator sandwiched between two electrodes sealed in liquid electrolyte) have been well addressed. However, the specific capacitance and high power properties of the former flexible solid-state devices still needs to be further improved.